Hot work tools are required to have a toughness such that they are resistant to impacts, since they are used in contact with high-temperature and hard workpieces. Conventionally, alloy tool steels, such as SKD61 which is a JIS steel grade, have been used for the hot work tool materials. Recently, further improved toughness has been required and thus alloy tool steels having modified composition of the SKD61 alloy tool steel have been proposed (see Patent Literatures 1 to 3).
Typically, hot work tool materials in an annealed state, which have a low hardness, are supplied to hot work tools manufacturers. The supplied materials are machined into shapes of the hot work tools and then quenched and tempered to have a specific hardness for use. After the adjustment of the hardness, they are typically subjected to finish machining. In some cases, the materials in the annealed state are quenched and tempered first, and then machined into the shapes of hot work tools together with the finish machining. The quenching is defined as an operation of heating a hot work tool material in an annealed state (or the hot work tool material after machined) to a temperature in an austenite region and then rapidly cooling it to cause a martensitic transformation. Therefore, a composition of the hot work tool material is adjusted such that the material can have the martensite structure by quenching.
It has been known that a toughness of the hot work tool can be improved when the hot work tool has a finer structure after the martensitic transformation. Specifically, it means that prior austenite grain size in the structure of the hot work tool is made fine. As a method for reducing the prior austenite grain size, it is effective to adjust a structure of the annealed hot work tool material before quenching. For example, it is proposed to make the structure “a mixed structure including, as observed at a magnification of 10,000, a region A of high carbide density where a number of carbides with 0.1 to 0.5 μm circle-equivalent diameter per 100 μm2 is 300 or more and a region B of low carbide density where a number of carbides with 0.1 μm to 0.5 μm circle-equivalent diameter is smaller 100 or more than that in the region A” (Patent Literature 4).